Hey Arthur, sorry to hear you're stuck with this frustrating FTP disconnect issue in your small business setup—let's walk through some targeted checks to get this sorted out:

• Verify FTP Authentication Settings
  Head into IIS Manager, navigate to your FTP site, and check the Authentication feature. Make sure the method you're using (like Basic Authentication) is enabled. Also, double-check that the Windows user account you're using to log into FTP has read/write permissions on the physical directory your FTP site maps to. If the account lacks filesystem access, the connection will drop right after login.

• Check FTP Authorization Rules
  Even if authentication passes, incorrect authorization rules can kill the connection instantly. In IIS, go to the Authorization Rules for your FTP site. Ensure there's a rule allowing your user (or their group) to access the site, with the appropriate permissions (Read/Write as needed). No valid authorization rule means IIS will reject the connection immediately.

• Validate Site Binding & Port Usage
  Confirm your FTP site is bound to the correct IP address—don't stick to just the loopback 127.0.0.1 if you need access from other LAN machines or remote Linux boxes. Bind it to your machine's static LAN IP or 0.0.0.0 (to listen on all interfaces). Also, check if port 21 is occupied by another program: run this command in Command Prompt:
  netstat -ano | findstr :21
  If you see a process ID (PID) that's not the IIS FTP service, you'll need to stop that program or switch your FTP port.

• Dig Into IIS FTP Logs
  Logs are your best troubleshooting tool here. By default, FTP logs are stored in C:\inetpub\logs\LogFiles in a folder starting with FTPSVC. Open the latest log file and look for failure status codes—common ones like 530 (Login failed) point to authentication/authorization issues, while other codes can reveal configuration problems. This will narrow down exactly where the connection breaks.

• Check Passive Mode Configuration
  IIS FTP uses passive mode by default, which requires a range of ports for data transfers. Even with the firewall off, misconfigured passive ports can cause immediate disconnects. Go to your FTP site's FTP Firewall Support settings, specify a port range (e.g., 50000-50100), and ensure this range is reserved for FTP. Remote clients like your Linux Mint machines rely on these ports to establish data connections, so misconfiguration here often causes drops.

• Confirm User Account Health
  Make sure the Windows user account you're using for FTP isn't locked out, expired, or restricted from local login. Check this in Computer Management > Local Users and Groups > Users. Right-click the account, go to Properties, and verify the "Account is locked out" and "Account expires" boxes aren't checked, and that the account has permission to log into the machine.

If none of these fixes work, try creating a brand new FTP site from scratch with minimal configuration—sometimes corrupted site settings can cause odd issues like this.

备注：内容来源于stack exchange，提问作者arthur brogard